docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # UI-Router for Angular  [![Build Status](https://travis-ci.org/ui-router/ng2.svg?branch=master)](https://travis-ci.org/ui-router/ng2)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/ui-router/angular.svg)](https://greenkeeper.io/)
+
 ### @uirouter/angular: State based routing for Angular (v2+)
 
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